This invention relates generally to making voltage measurements, and more particularly, to measuring voltages on a distribution line coupled to an electricity meter.
A watt-hour electricity meter typically measures several current and voltage inputs. DC isolation and an impedance are often provided to scale the currents down to appropriate levels for electronic measurement. Voltage inputs, however, are most economically measured by a simple resistor divider to neutral circuit (or some other voltage line if the service type has no neutral). The neutral is one of the AC distribution lines and is connected directly into the measurement electronics of the meter. This connection to the neutral line provides a direct low impedance path for transients from the AC lines to enter the meter electronics and disrupt operation or even damage the circuits.
Some known meters have voltage transformers in the voltage measurement circuit. These transformers perform multiple functions including scaling the voltage down to a level appropriate for measurement by an electronic circuit, and providing both DC isolation and some level of impedance to transients entering the meter electronics from the distribution lines. The transformers, however, are costly and heavy, failure prone, introduce measurement errors, consume significant power, and require a large amount of space in the meter.
Other known meters include resistors to scale the voltage down to a level appropriate for measurement by an electronic circuit. The resistors avoid some of the error types present in transformers. The resistors are also lower in cost, weight, power consumed, and size. The resistor method, however, provides no DC isolation or impedance between the neutral and the measurement electronics.
There is therefore provided, in one embodiment, a method for measuring a plurality of line voltages using a measurement circuit, the method including the steps of reducing each line voltage over a first impedance; providing a second impedance between each of the reduced voltages and a measurement circuit common reference point; coupling the common reference point to a voltage line N using a third impedance; and determining line voltage values using measurements of the reduced voltages. The above-described method provides impedance between voltage line N and measurement electronics and thus prevents transient voltages from entering the measurement circuit.